Process for manufacturing paper

ABSTRACT

The invention relates to a process for manufacturing paper and board comprising providing a suspension comprising cellulosic fibres and at least a sizing agent, dewatering said suspension thereby forming a paper-web, whereby an aromatic-containing cationic vinyl addition polymer, and an anionic polymer having a weight average molecular weight of up to about 500,000 selected from the group consisting of vinyl addition polymers and condensation polymers is added separately to the suspension.

[0001] This application is a continuation of application Ser. No.09/923,096, filed Aug. 6, 2001, which claims priority of EPO ApplicationNos. 00850195.9, filed Nov. 16, 2000, 00850135.5, filed Aug. 7, 2000,00850136.3, filed Aug. 7, 2000, and 00850137.1, filed Aug. 7, 2000, andU.S. Provisional Application No. 60/249,365, filed Nov. 16, 2000, No.60/223,367, filed Aug. 7, 2000, No. 60/223,368, filed Aug. 7, 2000, andNo. 60/223,369, filed Aug. 7, 2000.

[0002] The present invention refers to a process for manufacturing paperand board comprising the addition of two different polymers to anaqueous cellulose-containing suspension one being an aromatic-containingcationic vinyl addition polymer and the other an anionic vinyl additionpolymer having a weight average molecular weight of up to 500,000.

BACKGROUND

[0003] Internal sizing agents are usually added to the wet end of thepaper making process whereby the adsorption capability of the paper ofliquids is decreased. Commonly used internal sizing agents are sizingagents based on rosin derivatives and cellulose-reactive sizing agents,notably ketene dimers and acid anhydrides. Multipurpose office paperneed to be rather heavily sized in order to function properly in today'shigh speed reproducing machines. One way of attaining paper which isfully sized, i.e. having a cobb₆₀ number below 30 or measuring thecontact angle of a water droplet on the paper where angles larger the 80degrees after 10 seconds indicate good sizing, is to add more sizingagent to the suspension. However, the likelihood of ending up withrunability problems in the paper mill increases as well as theproduction costs.

[0004] Apart from the addition of sizing agents to the pulp suspension,dewatering and retention agents are also added to the suspension. As thename indicates, the latter agents enhance both dewatering and retentionof the pulp suspension. According to the present invention it hassurprisingly been found that sizing efficiency is improved by theaddition of at least two different types of polymers to the pulpsuspension which polymers simultaneously function as dewatering andretention agents.

[0005] According to the present invention it has been found thatspecifically improved sizing can be obtained by a process formanufacturing paper and board comprising providing a suspensioncomprising cellulose and at least a sizing agent, dewatering saidsuspension thereby forming a paper-web, whereby an aromatic-containingcationic vinyl addition polymer, and an anionic polymer having a weightaverage molecular weight of up to 500,000 selected from the groupconsisting of vinyl addition polymers and condensation polymers vinyladdition polymer are added separately to the suspension.

DETAILED DESCRIPTION OF THE INVENTION

[0006] The present invention is not restricted to specific types ofcellulose suspensions, but can be applied on cellulose suspensionscontaining virgin or recycled pulp and different fillers such as calciumcarbonate. The pH of the suspension may also vary from being acidic,which is the case if sizing agents derived from rosins are used, tobeing neutral or alkaline. If cellulose-reactive sizing agents are usedthe pH of the cellulose suspension is neutral to alkaline, i.e. in therange from about 5 up to about 10, which also makes it possible toinclude inorganic filler materials in the suspension, e.g. precipitatedcalcium carbonate and clays. The two different polymers are suitableadded to a fairly diluted lignocellulose-containing suspension commonlyreferred to as the thin stock having a concentration of from 0.1 up to3.0% by weight based on dry fibres.

[0007] The process is furthermore not dependent on the type of sizingagent added, thus, any sizing agent or mixture of sizing agents may bepresent in the cellulose suspension. Preferably, the cellulosesuspension contains cellulose-reactive sizing agents, normally presentin an amount of from 0.01 to 5% by weight based on dry fibres, and has apH value where the cellulose-reactive sizing agent still functionsproperly, i.e. a pH in the range from 5 up to 10. Suitablecellulose-reactive sizing agents are ketene dimers, ketene multimers,acid anhydrides, organic isocyanates, carbamoyl chlorides and mixturesthereof, where ketene dimers and acid anhydrides are preferred.

[0008] According to the present process an aromatic-containing cationicvinyl addition polymer and an anionic vinyl addition polymer having aweight average molecular weight of up to 500,000 is added to thecellulose suspension. Usually, the cationic polymer is added to thesuspension prior to the addition of the anionic polymer. Suitably, theaddition of the cationic polymer is followed by a shear stage or stages,whereas the anionic polymer is added after any stage providingsignificant shear but before the formation of the paper web.

[0009] Aromatic-Containing Cationic Vinyl Addition Polymer

[0010] The aromatic-containing cationic vinyl addition polymer may belinear or branched and contain monomers having anionic or potentiallyanionic groups as long as the overall charge of the polymer is cationic.However, the cationic polymer is preferably obtained by polymerising areaction mixture essentially free from monomers having anionic groups orgroups which can be rendered anionic in aqueous compositions. Thecationic polymer can be a homo polymer or a copolymer containingcationic aromatic monomers, cationic non-aromatic monomers and non-ionicmonomers, the latter also being non-aromatic. Suitably, the cationicvinyl addition polymer contains cationic aromatic monomers selected fromthe group consisting of acrylamide, (meth)acrylamide, acrylate and(meth)acrylate, whereby said cationic monomers preferably have at leastone aromatic group covalently linked to a nitrogen atom either direct orvia hydrocarbon groups which can have heteroatoms. Preferably, thearomatic-containing cationic vinyl addition polymer contains aromatic(meth)acrylamide and/or (meth)acrylate monomers which are present in thepolymer in an amount from about 2 molar % up to about 97 molar %. Thearomatic-containing cationic vinyl addition polymer is suitably obtainedby polymerising a cationic monomer or a reaction mixture containing amonomer mixture comprising a cationic monomer represented by the generalformula (I):

[0011] wherein R₁ is H or CH₃; R₂ and R₃ are independently from anothera hydrogen or an alkyl group having from 1 to 3 carbon atoms, usually 1to 2 carbon atoms; A₁ is 0 or NH; B₁ is an alkylene group having from 2to 8 carbon atoms, suitably from 2 to 4 carbon atoms, a hydroxypropylene group or a hydroxy ethylene group; Q is a substituentcontaining an aromatic group, suitably a phenyl or substituted phenylgroup, which can be attached to the nitrogen by means of an alkylenegroup usually having from 1 to 3 carbon atoms, suitably 1 to 2 carbonatoms, and preferably Q is a benzyl group (—CH₂—C₆H₅); and X is ananionic counterion, usually a halide like chloride. Examples of suitablemonomers represented by the general formula (I) include quaternarymonomers obtained by treating dialkylaminoalkyl (meth)acrylates, e.g.dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate anddimethylamino-hydroxypropyl (meth)acrylate, and dialkylaminoalkyl(meth)acrylamides, e.g. dimethylaminoethyl (meth)acrylamide,diethylaminoethyl (meth)acrylamide, dimethylaminopropyl(meth)-acrylamide, and diethylaminopropyl (meth)acrylamide, with benzylchloride. Preferred cationic monomers of the general formula (I) includedimethylaminoethylacrylate benzyl chloride quaternary salt,dimethylaminoethylmethacrylate benzyl chloride quaternary salt anddimethylaminopropyl(meth)acrylamide benzyl chloride quaternary salt.

[0012] The cationic vinyl addition polymer can be a homopolymer preparedfrom a cationic monomer having an aromatic group or a copolymer preparedfrom a monomer mixture comprising a cationic monomer having an aromaticgroup and one or more copolymerizable monomers. Suitable copolymerizablenon-ionic monomers include monomers represented by the general formula(II):

[0013] wherein R₄ is H or CH₃; R₅ and R₆ are each H or a hydrocarbongroup, suitably alkyl, having from 1 to 6, suitably from 1 to 4 andusually from 1 to 2 carbon atoms; A₂ is 0 or NH; B₂ is an alkylene groupof from 2 to 8 carbon atoms, suitably from 2 to 4 carbon atoms, or ahydroxy propylene group or, alternatively, A and B are both nothingwhereby there is a single bond between C and N (O═C—NR₅R₆). Examples ofsuitable copolymerizable monomers of this type include (meth)acrylamide;acrylamide-based monomers like N-alkyl (meth)acrylamides and N,N-dialkyl(meth)acrylamides, e.g. N-n-propylacrylamide, N-isopropyl(meth)acrylamide, N-n-butyl (meth)acrylamide, N-isobutyl(meth)acrylamide and N-t-butyl (meth)acrylamide; and dialkylaminoalkyl(meth)acrylamides, e.g. dimethylaminoethyl (meth)acrylamide,diethylaminoethyl (meth)acrylamide, dimethylaminopropyl (meth)acrylamideand diethylaminopropyl (meth)acrylamide; acrylate-based monomers likedialkylaminoalkyl (meth)acrylates, e.g. dimethylaminoethyl(meth)acrylate, diethylaminoethyl (meth)acrylate, t-butylaminoethyl(meth)acrylate and dimethylaminohydroxypropyl acrylate; and vinylamides,e.g. N-vinylformamide and N-vinylacetamide. Preferred copolymerizablenon-ionic monomers include acrylamide and methacrylamide, i.e.(meth)acrylamide, and the main polymer is preferably an acrylamide-basedpolymer.

[0014] Suitable copolymerizable cationic monomers include the monomersrepresented by the general formula (III):

[0015] wherein R₇ is H or CH₃; R₈ and R₉ are preferably a hydrocarbongroup, suitably an alkyl group having from 1 to 3 carbon atoms; R₁₀ canbe a hydrogen or preferably a hydrocarbon group, suitably an alkyl grouphaving from 1 to 8 carbon atoms, usually 1 to 2 carbon atoms; A₃ is 0 orNH; B3 is an alkylene group of from 2 to 4 carbon atoms, suitably from 2to 4 carbon atoms, or a hydroxy propylene group, and X is an anioniccounterion, usually methylsulphate or a halide like chloride. Examplesof suitable cationic copolymerizable monomers include acid additionsalts and quaternary ammonium salts of the dialkylaminoalkyl(meth)acrylates and dialkylaminoalkyl (meth)acrylamides mentioned above,usually prepared using acids like HCl, H₂SO₄, etc., or quaternizingagents like methyl chloride, dimethyl sulphate, etc.; anddiallyldimethylammonium chloride. Preferred copolymerizable cationicmonomers include dimethylaminoethyl (meth)acrylate methyl chloridequaternary salt, diallyldimethylammonium chloride and anddimethylaminopropyl(meth)acrylamide benzyl chloride quartenary salt.Copolymerizable anionic monomers like acrylic acid, methacrylic acid,itaconic acid, various sulfonated vinyl addition monomers, etc. can alsobe employed and, preferably, in minor amounts.

[0016] The cationic vinyl addition polymer can be prepared from amonomer mixture generally comprising from 1 to 99 mole %, suitably from2 to 50 mole % and preferably from 5 to 20 mole % of cationic monomerhaving an aromatic group, preferably represented by the general formula(I), and from 99 to 1 mole %, suitably from 98 to 50 mole %, andpreferably from 95 to 65 mole % of other copolymerizable monomers whichpreferably comprises acrylamide or methacrylamide ((meth)acrylamide),the monomer mixture suitably comprising from 98 to 50 mole % andpreferably from 95 to 80 mole % of (meth)acrylamide, the rest up to 100%preferably of compounds according to formula I and II.

[0017] Alternatively the cationic polymer can be a polymer subjected toaromatic modification using an agent containing an aromatic group.Suitable modifying agents of this type include benzyl chloride, benzylbromide, N-(3-chloro-2-hydroxypropyl)-N-benzyl-N,N-dimethylammoniumchloride, and N-(3-chloro-2-hydroxypropyl) pyridinium chloride. Suitablepolymers for such an aromatic modification include vinyl additionpolymers. If the polymer contains a tertiary nitrogen which can bequaternized by the modifying agent, the use of such agents usuallyresults in that the polymer is rendered cationic. Alternatively, thepolymer to be subjected to aromatic modification can be cationic, forexample a cationic vinyl addition polymer.

[0018] Usually the charge density of the cationic polymer is within therange of from 0.1 to 6.0 meqv/g of dry polymer, suitably from 0.2 to 4.0and preferably from 0.5 to 3.0. The weight average molecular weight ofthe cationic polymer is usually at least about 500,000, suitably aboveabout 1,000,000 and preferably above about 2,000,000. The upper limit isnot critical; it can be about 30,000,000, usually 20,000,000 andsuitably 10,000,000.

[0019] The cationic vinyl addition polymer can be added into thesuspension in amounts which can vary within wide limits depending on,inter alia, type of suspension, salt content, type of salts, fillercontent, type of filler, point of addition, etc. Generally the cationicvinyl addition polymer is added in an amount that give better sizing,dewatering and retention than is obtained when not adding it providedthe anionic vinyl addition polymer is added. The cationic polymer isusually added in an amount of at least 0.002%, often at least 0.005% byweight, based on dry pulp, whereas the upper limit is usually 1.0% andsuitably 0.5% by weight.

[0020] Anionic Vinyl Addition Polymer

[0021] Further to the above described aromatic-containing cationic vinyladdition polymer, an anionic polymer having a weight average molecularweight of up to 500,000 selected from the group consisting of vinyladdition polymers and condensation polymers is added to the cellulosesuspension. The anionic polymer can be linear, branched or cross-linked,yet suitably essentially linear, and usually water-soluble orwater-dispersable. The anionic polymer may furthermore be a homopolymeror a copolymer containing at least two different types of monomers.Preferably, the anionic polymer is a vinyl addition polymer having aweight average molecular weight of up to 500,000. Suitable anionic vinyladdition polymers are polymers obtained from a reaction mixturecomprising vinylic unsaturated monomers, preferably vinylic unsaturatedaromatic containing monomers, having one or more anionic groups orgroups rendered anionic in aqueous solutions, suitably at least onesulphonate group. Examples of anionic groups attached to vinylicunsaturated monomers are phosphate groups, phosphonate groups, sulphategroups, sulphonic acid groups, sulphonate groups, carboxylic acidgroups, carboxylate groups such as acrylic acid, methacrylic acid, ethylacrylic acid, crotonic acid, itaconic acid, maleic acid or saltsthereof, alkoxide groups, maleic acid groups and phenolic groups, i.e.hydroxy-substituted phenyls and naphthyls. Groups carrying an anioniccharge are usually salts of an alkali metal, alkaline earth or ammonia.The anionic vinyl addition polymer may also in some extent containcationic groups such as monomers having cationic groups, though,preferable the only ionic groups present in the vinyl addition polymerare anionic. Preferably, the anionic groups are linked to aromaticvinylic (ethylenically) unsaturated monomers such as styrene, i.e.styrene sulphonate. If the anionic vinyl addition polymer is acopolymer, said polymer can be obtained from a reaction mixturecomprising non-ionic vinylic unsaturated monomers, e.g. acrylamide,(meth)acrylamide. The anionic vinyl addition polymer may comprise fromabout 20 mole % up to about 100 mole % of anionic monomers containing atleast one anionic charge.

[0022] Suitable anionic condensation polymers having a weight averagemolecular weight of up to 500,000 are condensates of an aldehyde such asformaldehyde with one or more aromatic compounds containing one or moreanionic groups, and optional other co-monomers useful in thecondensation polymerization such as urea and melamine. Examples ofsuitable aromatic compounds containing anionic groups comprises benzeneand naphthalene-based compounds containing anionic groups such asphenolic and naphtholic compounds, e.g. phenol, naphthol, resorcinol andderivatives thereof, aromatic acids and salts thereof, e.g. phenylic,phenolic, naphthylic and naphtholic acids and salts, usually sulphonicacids and sulphonates, e.g. benzene sulphonic acid and sulphonate, xylensulphonic acid and sulphonates, naphthalene sulphonic acid andsulphonate, phenol sulphonic acid and sulphonate. Examples of suitableanionic condensation polymers include anionic benzene-based andnaphthalene-based condensation polymers, preferablynaphthalene-sulphonic acid based and naphthalene-sulphonate basedcondensation polymers.

[0023] The weight average molecular weight of the anionic polymer is upto 500,000, suitably up to 250,000. Preferred ranges of the weightaverage molecular weight are from about 10,000 up to about 100,000,preferably from about 15,000 up to about 75,000, suitably from about15,000 up to about 45,000, and most preferably from about 25,000 up toabout 40,000.

[0024] The anionic polymer can have a degree of anionic substitution(DSA) varying over a wide range dependent on, inter alia, the type ofpolymer used; DSA is usually from 0.01 to 2.0, suitably from 0.02 to 1.8and preferably from 0.025 to 1.5; and the degree of aromaticsubstitution (DSQ) can be from 0.001 to 1.0, usually from 0.01 to 1.0,suitably from 0.02 to 0.7 and preferably from 0.025 to 0.5. In case theanionic polymer contains cationic groups, the degree of cationicsubstitution (DS_(c)) can be, for example, from 0 to 0.2, suitably from0 to 0.1 and preferably from 0 to 0.05, the anionic polymer having anoverall anionic charge. Usually the anionic charge density of theanionic polymer is within the range of from 0.1 to 6.0 meqv/g of drypolymer, suitably from 0.5 to 5.0 and preferably from 1.0 to 5.0.

[0025] The anionic polymer can be added to the suspension in amountswhich can vary within wide limits depending on, inter alia, type ofstock, salt content, type of salts, filler content, type of filler,point of addition, etc. Generally the anionic polymer is added in anamount that give better sizing, dewatering and retention than isobtained when not adding the anionic polymer provided the cationic vinyladdition polymer is added. The anionic polymer is usually added in anamount of at least 0.001%, often at least 0.005% by weight, based on drypulp, whereas the upper limit is usually 3.0% and suitably 1.0% byweight.

[0026] According to one preferred embodiment of the present inventionthe aromatic-containing cationic vinyl addition polymer can be providedas an aqueous composition, suitably aqueous solution, preferablycomprising further cationic polymers, for example synthetic cationicpolymers and naturally occurring polymers. Suitable synthetic cationicpolymers cationic are vinyl addition polymers such as acrylamide basedpolymers or acrylate based polymers. Other synthetic cationic polymersinclude cationic condensation polymers like epihalohydrin polymers, e.g.polymers formed by reacting aliphatic amines and epichlorohydrine,polyamideamine polymers, polyethyleneimine polymers. Preferred naturallyoccurring cationic polymers as cationic polysaccharides, particularlycationic starch and aromatic substituted cationic starch. The aqueoussolution preferably contains the aromatic-containing cationic vinyladdition polymer in a predominant amount, i.e. at least 50% by weight,even though effects are present at considerably lesser amounts, down toamount at least 10% by weight. The further cationic polymers referred toin this paragraph may also be added separately.

[0027] According to yet another preferred embodiment of the presentinvention inorganic anionic microparticulate materials like anionicsilica-based particles, polysilicic acid and clays of the smectite typeare added to the suspension. The inorganic anionic microparticulatematerial can be added separately to the suspension or is preferablycomprised in an aqueous composition also comprising the anionic polymer.

[0028] The invention is further illustrated in the following exampleswhich, however, are not intended to limit the same. Parts and % relateto parts by weight and % by weight based on dry fibres, respectively,unless otherwise stated. All compound added to the furnish arecalculated as dry material, if not otherwise indicated. In the examples,a good retention is shown by a low turbidity value in the white water,i.e. more fines and filler are retained in the formed sheet. A turbidityvalue under 120 is acceptable and a value under 90 is in this set ofexperiment excellent. The dewatering figure should also be low. Thesizing of the paper was measured by the contact angle of a water dropleton the paper. Contact angles larger the 80 degrees after 10 seconds areindicating a good sizing.

EXAMPLE 1

[0029] The pulp (at 3%) used was a 80/20 mixture of hardwood/softwoodkraft. Ground calcium carbonate filler (GCC) was added to the pulp, to afiller concentration of 40% on dry solids. The resulting furnish wasdiluted to 0.3% before additional chemicals were added. The chemicaladditions are expressed as % on dry solids in the furnish.

[0030] In this example two furnishes was used one having a lowconductivity of 500 μS/cm (furnish I), the other having a highconductivity of 4.0 RS/cm (furnish II). The conductivity was adjusted byaddition of sodium sulphate. A dispersion containing a conventionalketene dimer sizing agent and 1% cationic starch were added to thefurnishes. Subsequent to these additions, either 0.1% of an aromaticcationic polyacrylamide having benzyldimethylammonium groups (A-PAM) or0.1% of a conventional non-aromatic cationic polyacrylamide (C-PAM) wasadded prior to the addition of either 0.1% of a silica sol or 0.1% of ananionic polystyrene sulphonate having a weight average molecular weightof 70,000 (PSS). The added amounts of compounds are indicated in table Iand II. The retention and dewatering properties of the formed furnisheswere evaluated by measuring the dewatering time using a Dynamic DrainageAnalyser (DDA-unit). A lower value in this test means better dewateringefficiency. The retention was evaluated by measuring the turbidity ofthe white water with a Nephelometer 156 from Novasine. A lower turbidityvalue signifies higher retention of solids in the DDA-unit. Moreover,the sizing of the formed, dried and cured paper was evaluated bymeasuring the contact angle of water after 10 seconds utilising aDynamic Absoption and contact angle tester from Fibro Systems (DAT). Ahigher value of the contact angle means better sizing efficiency. TABLEI Furnish II (high conductivity) Amount of Type of Contact added ketenecationic Type of angel (10 dimer/[kg/t dry polyacryla- anionicDewatering/ sec./[de- test pulp] mide compound Turbidity [sec.] grees]blank* 0.2 none none 390 7.8 below 10 1 0.2 C-PAM silica sol 91 6.9229.6 2 0.2 A-PAM PSS 47 4.54 44.6 3 0.3 C-PAM silica sol 90 6.64 80.8 40.3 A-PAM PSS 43 4.47 84.6 5 0.4 C-PAM silica sol 90 6.77 89.9 6 0.4A-PAM PSS 47 4.47 94.4

[0031] As shown by table I, the addition of an aromatic-modifiedcationic vinyl addition polymer and an anionic vinyl addition polymerwith a weight average molecular weight of up to 500,000 significantlyincreases not only dewatering and retention but also the sizingefficiency. TABLE II Furnish I (low conductivity) Amount of added Typeof Contact ketene cationic Type of angle (10 dimer/[kg/t polyacryla-anionic Dewatering/ sec./[de- test dry pulp] mide compound Turbidity[sec.] grees] blank* 0.3 none none 420 5.6 35 1 0.3 C-PAM silica-sol 1004.8 83.3 2 0.3 A-PAM PSS 76 3.5 87.8

EXAMPLE 2

[0032] The furnish used was the same as used in example 1, however, inthis example the furnish was adjusted to a conductivity of 400 μS/cm

[0033] The sizing dispersion as used in example 1 was added to thefurnish followed by the addition of cationic starch. The dosage for thesize was 0.03% (calculated as active ketene dimer on dry furnish) andfor the cationic starch 1.0%. Subsequent to these additions, 0.1% of anaromatic cationic polyacrylamide having benzyldimethylammonium groupswas added prior to the addition of 0.07% of an anionic polystyrenesulphonate having different weight average molecular weights asindicated in table III. The added amounts of compounds are indicated intable III. The retention and dewatering properties of the formedfurnishes were evaluated by measuring the dewatering time using aDDA-unit. The retention was evaluated by measuring the turbidity of thewhite water with a Nephelometer 156 from Novasine. Moreover, the sizingof the formed, dried and cured paper was evaluated by measuring thecontact angle of water after 10 seconds utilising a DAT equipment. TABLEIII Weight average molecular Contact angle weight of the (10 test PSSTurbidity Dewatering/[sec.] sec./[degrees] blank* none 125  5.4  below30 1  35,000 56 4.89 92.7 2 220,000 39 3.49 82.1 3 780,000 30 3.17 69.2

[0034] Tests 1 and 2 are according to the present invention, i.e. theanionic vinyl addition polymer having a weight average molecular weightof up to 500,000. As can be seen in table III, the sizing efficiency issignificantly increased while the turbidity and dewatering performanceare high with regard to tests 1 and 2 compared to the blank. Inaddition, comparing test 3 with tests 1 and 2 (the latter two accordingto the invention), the sizing efficiency is much higher, while theturbidity value still indicates excellent retention. What is more, acontacting angle of 69.2 as obtained in test 3 is not an acceptablesizing degree. Thus, the overall performance of test 1 and 2 in respectof retention, dewatering and not least sizing clearly outperform test 3.

1. A process for manufacturing paper and board comprising providing asuspension comprising cellulosic fibres and at least a sizing agent,dewatering said suspension thereby forming a paper-web, wherein i) anaromatic-containing cationic vinyl addition polymer is added to thesuspension in an amount of from about 0.005% by weight up to about 1.0%by weight based on dry pulp, and ii) an anionic polymer having a weightaverage molecular weight of up to about 500,000 selected from the groupconsisting of vinyl addition polymers and condensation polymers is addedto the suspension in an amount of from about 0.001% by weight up toabout 3.0% by weight based on dry pulp, whereby the aromatic-containingcationic vinyl addition polymer and the anionic polymer are addedseparately to the suspension.
 2. A process according to claim 1, whereinthe anionic polymer has a weight average molecular weight in the rangefrom about 10,000 up to about 100,000.
 3. A process according to claim2, wherein the weight average molecular weight is in the range fromabout 15,000 up to about 75,000.
 4. A process according to claim 1,wherein the anionic polymer comprises aromatic monomers havingsulphonate groups.
 5. A process according to claim 1, wherein theanionic polymer is a vinyl addition polymer having a weight averagemolecular weight of up to about 500,000.
 6. A process according to claim5, wherein the anionic vinyl addition polymer comprises aromaticmonomers.
 7. A process according to claim 6, wherein the aromaticmonomers have at least one sulphonate group.
 8. A process according toclaim 5, wherein the anionic vinyl addition polymer is polystyrenesulphonate.
 9. A process according to claim 1, wherein thearomatic-containing cationic vinyl addition polymer has a weight averagemolecular weight of at least about 500,000.
 10. A process according toclaim 1, wherein the cationic vinyl addition polymer is prepared from areaction mixture comprising from about 1 up to 99 mole % of a cationicmonomer having an aromatic group.
 11. A process according to claim 11,wherein the cationic monomer having an aromatic group is represented byformula (I)

wherein R₁ is H or CH₃; R₂ and R₃ are independently from another ahydrogen or an alkyl group having from 1 to 3 carbon atoms; A₁ is 0 orNH; B₁ is an alkylene group having from 2 to 8 carbon atoms; Q is asubstituent containing an aromatic group; and X is an anioniccounterion.